Most of sulfur oxides (SOx) contained in flue gas emitted from a boiler of a power plant and the like are sulfur dioxide (SO2), but some of them are converted to sulfur trioxide (SO3) due to catalytic function of metal-oxides carried on denitrification catalyst or coexisting incinerated ashes. Due to high reactivity and corrosivity of SO3, neutralization thereof has been generally carried out in the related art. Such neutralization is carried out by injecting ammonia to a flue gas duct so as to prevent deterioration of equipment. When combusted in a boiler, sulfur (S) content derived from fuel of a thermal power plant is oxidized by catalyst components in the fuel or catalysts of a denitrification device and is converted from sulfur dioxide (SO2) to sulfur trioxide (SO3). Concentration of this converted SO3 sometimes reaches about 30 to 50 ppm at a maximum in the flue gas of a coal combustion boiler. In oil combustion-heavy fuel, the concentration of this converted SO3 sometimes reaches about 180 ppm at a maximum, though it depends on a condition of contamination in the boiler or a condition of combustion. SO3 in the flue gas may cause problems such as a corrosion of equipment or a clogging due to increased ash-adhesiveness and the like, in cryogenic equipment which is in a downstream of an air heater. Such a problem may lead to increasing maintenance costs of a power plant user. Furthermore, SO3 is known as a typical component that causes blue smoke from a stack. In a case of thick blue smoke, there may be no choice but to stop the plant.
Therefore, examples of a method for removing this SO3 in the related art include the following proposals. That is, spraying CaCO3, Ca(OH)2, CaO, and the like to the flue gas duct in addition to introducing ammonia thereto, and spraying Active Carbon (AC; a porous impurity-absorbent containing carbon as a principal component) to remove SOx (see Patent Literatures 1 and 2).
Furthermore, Patent Literature 3 discloses an air pollution control system including a denitrification device, an air heater, a heat-recovery unit, a precipitator, and a desulfurization device. Herein, the denitrification device removes nitrogen oxides of flue gas from a boiler. The air heater recovers heat of the flue gas which has passed through the denitrification device. The heat-recovery unit recovers the heat of the flue gas and is used for raising a temperature of the flue gas emitted from a stack. The precipitator removes dust in the flue gas after the heat recovery. The desulfurization device removes sulfur oxides in the flue gas after the dust removal. With regard to the precipitator, a system provided before the heat-recovery unit may also be used. A typical example of the desulfurization device includes a wet-type desulfurization device which removes the sulfur oxides in the flue gas by bringing the flue gas into gas-liquid contact with an absorbent and the like containing slurry in which limestone (calcium carbonate, CaCO3) and other components are suspended. The device disclosed in Patent Literature 3 includes a means for controlling a quantity of a solid particle to be supplied to a charge-adsorption means based on detected values of concentration of SO3 in a flue gas outlet of the desulfurization device.